1. Field of the Invention
This invention relates to electrical switching apparatus and, more particularly, to circuit interrupters, such as, for example, aircraft or aerospace circuit breakers providing arc fault protection. The invention also relates to methods of actuating a test function of an electrical switching apparatus, such as, for example, an arc fault test of an aircraft or aerospace circuit breaker.
2. Background Information
Electrical switching apparatus include, for example, circuit switching devices; circuit interrupters, such as circuit breakers; network protectors; contactors; motor starters; motor controllers; and other load controllers.
Circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition or a relatively high level short circuit or fault condition. In small circuit breakers, commonly referred to as miniature circuit breakers, used for residential and light commercial applications, such protection is typically provided by a thermal-magnetic trip device. This trip device includes a bimetal, which heats and bends in response to a persistent overcurrent condition. The bimetal, in turn, unlatches a spring powered operating mechanism, which opens the separable contacts of the circuit breaker to interrupt current flow in the protected power system.
Subminiature circuit breakers are used, for example, in aircraft or aerospace electrical systems where they not only provide overcurrent protection but also serve as switches for turning equipment on and off. Such circuit breakers must be small to accommodate the high-density layout of circuit breaker panels, which make circuit breakers for numerous circuits accessible to a user. Aircraft electrical systems, for example, usually consist of hundreds of circuit breakers, each of which is used for a circuit protection function as well as a circuit disconnection function through a push-pull handle.
Typically, subminiature circuit breakers have provided protection against persistent overcurrents implemented by a latch triggered by a bimetal responsive to I2R heating resulting from the overcurrent. There is a growing interest in providing additional protection, and most importantly arc fault protection.
During sporadic arc fault conditions, the overload capability of the circuit breaker will not function since the root-mean-squared (RMS) value of the fault current is too small to actuate the automatic trip circuit. The addition of electronic arc fault sensing to a circuit breaker can add one of the elements required for sputtering arc fault protection—ideally, the output of an electronic arc fault sensing circuit directly trips and, thus, opens the circuit breaker. See, for example, U.S. Pat. Nos. 6,710,688; 6,542,056; 6,522,509; 6,522,228; 5,691,869; and 5,224,006.
Common methods of actuating a test function on, for example, a circuit breaker, include employing a mechanical pushbutton switch. See, for example, U.S. Pat. Nos. 5,982,593; 5,459,630; 5,293,522; 5,260,676; and 4,081,852. However, such mechanical mechanisms often fail due to mechanical stress and may be actuated by mistake. Furthermore, such mechanical mechanisms, when employed on a relatively small circuit breaker, such as, for example, a sub-miniature circuit breaker, are of relatively large size.
Proximity sensors include, for example, Hall effect sensors. These sensors, used in automatic metal detectors, change their electrical characteristics when exposed to a magnet. Usually, such sensors have three wires for supply voltage, signal and ground.
Installation of arc fault circuit breakers in panels of aircraft (e.g., without limitation, optimized panels of F-15, F-16 or F-18 combat helicopters) provides little space to test such circuit breakers using an arc fault tester. When using the arc fault tester, the panel must be opened to access the rear terminals of the arc fault circuit breaker, the load connection to the arc fault circuit breaker must be disconnected, and the arc fault tester must be manually run.
U.S. Patent Application Publication No. 2006/0125582 discloses an aircraft or aerospace arc fault circuit breaker including a Hall effect sensor disposed within a housing and proximate a rear opening thereof. A target, such as a magnetic tool or magnetic wand, is inserted within the rear opening to actuate the Hall effect sensor. When the sensor changes state, this informs a processor that a test function has been initiated. The processor, then, responsively outputs a pulse stream signal that simulates an arcing event into the input stage of an arc fault detector, which trips the circuit breaker.
Aircraft or aerospace panels, which are typically dielectrically coated, obstruct testing from the rear of such panels when attempting to insert the magnetic target within the rear housing opening of the circuit breaker to actuate the Hall effect sensor. Hence, the panel must be removed to initiate the test function. However, test personnel must exercise extreme caution since the rear line and load terminals of the circuit breaker remain energized.
There is room for improvement in electrical switching apparatus employing a test function and in methods of actuating a test function of a panel-mounted electrical switching apparatus.